Silicoaluminophosphate (SAPO) molecular sieves have been used as an absorbent and a catalyst. As a catalyst, the SAPO molecular sieves have been used in the processes of fluidized catalytic cracking, hydrocracking, isomerization, polymerization, conversion of alcohols or ethers, and alkylation of aromatic compounds. In particular, in recent years, along with the advent of high oil prices, methods for using a SAPO molecular sieve in the process of converting alcohols or ethers into light olefins are drawing significant attention.
A SAPO molecular sieve was first synthesized by the Union Carbide Co. [U.S. Pat. Nos. 4,310,440 and 4,440,871]. In particular, the same company published documents on how light olefins (C2 to C4 olefins) are prepared from oxygen-including compounds including methanol by using a SAPO-34 catalyst [U.S. Pat. Nos. 4,440,871 and 4,499,327], and major petroleum companies such as UOP, Exxon Mobil, and the like have developed SAPO-34 molecular sieve catalysts as a catalyst for methanol to olefin (MTO) reaction.
In the methanol-to-olefin (MTO) reaction, as another method of enhancing the activity of the catalyst while increasing the yield of light olefins, aluminophosphate molecular sieves substituted with various metals were published in U.S. Pat. Nos. 5,126,308 and 5,191,141, and silicon, magnesium, cobalt, zinc, iron, nickel, manganese, chromium, or mixed components thereof were used as metal components in the documents. In this case, it was reported that silicon is the most favorably used metal and the activity and durability of the catalyst is excellent when particles having a molecular sieve crystal size of less than 1 μm are present in an amount of 50% and particles having a molecular sieve crystal size of more than 2.0 μm are present in an amount of less than 10%, with respect to the whole system and the content of silicon is limited to 0.005 to 0.05 in terms of molar fraction. Further, U.S. Pat. No. 6,207,872 by the same company is an improvement patent of U.S. Pat. No. 5,126,308. It was reported in the patent document that the yield of light olefins is increased when the molar fraction of molecular components is in a range of 0.02 to 0.08 based on the aluminophsphate molecular sieve substituted with the same metal components and the molecular sieve crystal size is 0.1 μm or more. As described above, it is already known that the reaction performance is improved when the crystal size is small. However, when the crystal size is small, it is disadvantageous in that the preparation yield of catalyst powder through filtration and drying is decreased and as a result, costs of preparing a shaped catalyst from a powdered catalyst are increased.
As a specific method of preparing a SAPO molecular sieve, usually, humed silica, silica sol, and the like are used as a silicon precursor, pseudoboehmite, aluminum isoprooxide, and the like are used as an aluminum precursor, and phosphoric acid is used as a precursor of phosphor, generally in the SAPO molecular sieve. As an organic templates which is used to form the backbone of the molecular sieve which is the most important, tetraethylammonium hydroxide, morpholine, dipropylamine, isopropylamine, diethanolamine, triethylamine, diethylamine, cyclopentylamine, aminomethyl cyclohexane, piperidine, cyclohexylamine, tri-ethyl hydroxyethylamine, pyridine, and the like are known [Korean Patent No. 699,654 and Korean Patent No. 699,650]. However, when the other organic templates except for tetraethylammonium hydroxide are used alone, the crystallinity is lowered and the crystal size is increased too much, and thus it is disadvantageous in that the reaction activity is relatively decreased. Meanwhile, in Korean Patent No. 989,127, it was suggested that the crystallinity and reactivity of the catalyst may be significantly enhanced by using these templates in mixtures, but what is described above is limited to the preparation of powdered catalysts and is not appropriate for use in a circulating-fluidized bed reactor.
In order to commercialize the light olefin process, it is very important to develop a shaped catalyst having excellent sphericity and high strength while having an appropriate size in a circulating-fluidized bed reactor which may regenerate the catalyst continuously. The circulating-fluidized bed reactor is effective in controlling a constant reaction temperature, and particularly, the reaction heat of the exothermic reaction, compared to a fixed bed reactor and advantageous in that productivity is enhanced by contacting reactants with the catalyst safely and uniformly. However, stress of catalytic materials resulting from the high temperature and flow rate of the circulating-fluidized bed reactor acts so significantly that due to attrition of the catalyst in the circulating-fluidized bed reactor, fine particles are produced, thereby leading to loss of the catalyst. Further, fine particles are released while being incorporated into products, and thus a product separation filter is clogged to hamper the reactor from being operated normally, which is problematic. Therefore, in the development of a SAPO catalyst for preparing light olefins, it is particularly important to develop a microsphere SAPO catalyst having excellent abrasion resistance along with optimization of reactivity.